1. Field of the Invention
The present invention relates to a digital graphic equalizer, and, more particularly, to a digital graphic equalizer which has a boost characteristic and an attenuation characteristic which are symmetrical to each other with respect to a reference level, and small S/N deterioration.
2. Description of the Related Art
Audio systems typically use a graphic equalizer which can freely change the frequency characteristic of a reproduced signal to create the desired reproduction sound field characteristic. In particular, due to the recent development of the digital signal processing technology, a digital graphic equalizer which uses a digital filter becomes popular. The circuit functions of such a digital graphic equalizer are generally accomplished on a software basis using a DSP (digital signal processor).
FIG. 1 shows one example of a digital graphic equalizer in which a plurality of equalizers 51.sub.1 to 51.sub.n each constituted by an IIR (Infinite Impulse Response) digital filter are cascade-connected. By shifting the center frequencies of the equalizers 51.sub.1 to 51.sub.n slightly from one another, the equalizers 51.sub.1 to 51.sub.n as a whole achieve the mixed boost characteristic that is shown in FIG. 2.
FIG. 3 shows one example of a digital graphic equalizer in which a plurality of band pass filters 61.sub.1 to 61.sub.n each constituted by an IIR digital filter are connected in parallel. In this case too, the band pass filters 61.sub.1 to 61.sub.n as a whole achieve the mixed boost characteristic that is shown in FIG. 4 by shifting the center frequencies of the band pass filters 61.sub.1 to 61.sub.n slightly from one another.
This type of graphic equalizer executes attenuating as well as boosting. It is desirable that the boost characteristic and the attenuation characteristic form curves which are symmetrical to each other with respect to a reference level. With regard to the cascade-connected type digital graphic equalizer shown in FIG. 1, the curves of the boost characteristic and the attenuation characteristic are approximately symmetrical to each other with respect to the reference level. However, the mixed amplitude varies greatly and great attenuation must be given previously to avoid saturation, particularly, at a full boosting time, resulting in deterioration of the S/N ratio.
The parallel type digital graphic equalizer shown in FIG. 3 cannot have the boost characteristic and the attenuation characteristic which are symmetrical to each other. This will be explained more specifically with reference to the case where the digital graphic equalizer in FIG. 3 is constituted by band pass filters 61.sub.1 to 61.sub.n. A transfer function at the boosting time (hereinafter referred to as "boost-mode transfer function"), H.sub.BOOST (Z), is expressed by EQU H.sub.BOOST (Z)=1+H.sub.BPF1 (Z)+H.sub.BPF2 (Z)+ . . . +H.sub.BPFn (Z)(1)
where H.sub.BPF1 (Z) to H.sub.BPFn (Z) are transfer functions of the individual band pass filters. Assuming that H.sub.ATTEN (Z) is a transfer function at the attenuating time (hereinafter referred to as "attenuation-mode transfer function") which provides a frequency characteristic vertically symmetrical to the frequency characteristic given by the boost-mode transfer function H.sub.BOOST (Z), H.sub.ATTEN (Z) and H.sub.BOOST (Z) should have a relationship of H.sub.ATTEN (Z)=1/H.sub.BOOST (Z). Thus, ##EQU1##
If this transfer function H.sub.ATTEN (Z) could be realized at the attenuating time, the boost characteristic and attenuation characteristic would form symmetrical curves with respect to the reference level. Since the circuit structure shown in FIG. 3 is designed to provide the transfer function H.sub.BOOST (Z), however, this circuit structure in FIG. 3 alone cannot provide the boost characteristic and attenuation characteristic that form symmetrical curves with respect to the reference level.
As one solution to this problem, the phases of the outputs of the individual band pass filters in FIG. 3 are inverted and the resultant values are then added (i.e., subtracted). The transfer function in this case, H'.sub.ATTEN (Z) is expressed by EQU H'.sub.ATTEN (Z)=1-H'.sub.BPF1 (Z)-H'.sub.BPF2 (Z)- . . . -H'.sub.BPF2 (Z)(3)
where H'.sub.BPF1 (Z) to H'.sub.BPFn (Z) are new transfer functions of the individual band pass filters.
By re-setting the transfer functions H'.sub.BPF1 (Z) to H'.sub.BPFn (Z) of the individual band pass filters 61.sub.1 to 61.sub.n so that the transfer function H'.sub.ATTEN (Z) approximately equals the attenuation-mode transfer function H.sub.ATTEN (Z), i.e., H'.sub.ATTEN (Z).apprxeq.H.sub.ATTEN (Z), therefore, even the circuit in FIG. 3 can provide the attenuation-mode and boost-mode characteristics that form approximately symmetrical curves.
However, the above scheme involves complex computations to obtain new transfer functions H'.sub.BPF1 (Z) to H'.sub.BPFn (Z), making it difficult to obtain good approximation. In the case where the interval between center frequencies of adjoining bands is narrow and the foot portions of frequency characteristics overlap, in particular, if one band characteristic is to be changed, a satisfactory approximation is not obtained unless the characteristics of the band pass filters of the adjoining bands are also changed.
Further, the above scheme requires a coefficient table covering all the combinations of the levels of the entire bands, so that as the number of bands increases, the amount of data becomes enormous. This increases the memory capacity for storing such vast amount of data and increases the load of a DSP or a control microprocessor which performs the computations. This scheme is therefore hardly practical.